In line with recent advent of the information-intensive society, visualized displays have been given much more weight as means for connecting human being with machines by the informational exchange. Specifically, small-sized flat displays which can work by low electric power have been widely used in electronic calculators, watches, toys and notebook computers, and so on.
Among the various flat displays, a liquid crystal display(hereinafter referred to as `LCD`) is fallen within a passive light-emitting display. That is, image is created on a screen by changing exterior light path or by modulating density of passing light instead of direct light emission. Thus, LCD has various advantages that: electric power consumption is low, its size is small, operation is possible under a relatively low voltage, and image on a screen does not disappear during incidence of bright light onto a screen. Therefore, LCD has enjoyed the distinction as a flat display which has been widely used in the art.
The LCD is further classified into a passive-driven display and an active-driven display, depending on its driving mode: Passive-driven LCD drives liquid crystal only by electrodes without employing a specific wafer in a pixel, and has advantages of simplicity and inexpensiveness, while it has also disadvantages that improper ratio of light and shade gives rise to darkness, and low response speed results in afterimage. On the contrary, active-driven LCD developed by combining the conventional liquid crystal technology with semiconductor technology has excellent properties of screen display, in light of high definition, wide view angle, improved color quality and use of relatively quick liquid crystal.
Recently developed active-driven LCDs have a structure that a transparent electrode is formed in each pixel on a glass plate, and liquid crystal is coated on the electrode. Thus, application of voltage to the transparent electrode leads to the change in polarization of the liquid crystal on the electrode, which, in turn, modulates brightness of each pixel. Control of the transparent electrode is accomplished by a pixel switching device prepared on the glass plate.
On the other hand, thin film-type transistor which is a pixel switching device of an active-driven LCD, being made of polycrystalline silicon, amorphous silicon, etc., has been proven less satisfactory in the sense that it has bad electrical properties, compared to the conventional devices prepared with single crystalline silicon. Nevertheless, the polycrystalline silicon has been employed to manufacture a device on a glass plate accompanying with the improvement of its properties, since single crystalline silicon can not be grown on a glass plate, which is essentially required for light transmission in the course of manufacturing a transparent LCD. However, during the preparation of substrate using conventional glass plates, deformation of the glass may occur at a high temperature. Accordingly, quartz of high price has to be used in order to carry out the process at a high temperature which is a critical step for the improvement of electrical properties. However, the devices prepared with quartz have still worse properties than devices made of single crystalline silicon in light of operation speed, etc.
Consequently, if single crystalline silicon substrate can be used in the manufacture of LCDs, the following advantages would be expected: integration of driving circuit and extra circuit is facilitated; recently developed integration techniques can be employed to manufacture small-sized devices; high resolution and high level of integration can be accomplished; and, electric power consumption can also be reduced.
In general, a silicon on insulator(SOI) device which is prepared in a single crystalline silicon region on oxidized silicon layer, has merits of good electrical insulation properties, low leakage current and small parasitic capacity, compared to wafers prepared simply on single crystalline silicon, though SOI substrates used therein are more expensive than common silicon substrates.